摘要
镁铁质岩石出露有限,超镁铁岩出露厚度大是西藏雅鲁藏布江日喀则蛇绿岩的典型特征.20世纪80年代中国科学家及中法合作研究以来,积累了大量岩石学、地球化学和同位素年代学资料,阐明了该蛇绿岩的基本地质特征,但是其形成背景一直是争论的焦点.通过对日喀则蛇绿岩西段白马让岩体地质填图和构造地质学的研究,从岩体中厘定出2条糜棱岩化蛇纹岩带,发现岩体具有穹窿形的几何学结构.研究发现岩体的岩石组合及构造变形特征类似于大洋核杂岩构造:岩体核部发育高温流动构造的方辉橄榄岩相当于大洋核杂岩核部的大洋岩石圈地幔;方辉橄榄岩两侧的糜棱岩化蛇纹石带相当于拆离断层带,拆离断层内发育有强烈变形的辉绿岩、细粒辉长岩和碳酸岩;拆离断层之上的辉绿岩和玄武岩则相当于拆离断层的上盘.该成果对于重新理解雅鲁藏布江蛇绿岩成因、多样性,在造山带厘定和开展大洋核杂岩研究,以及在造山带中研究岩浆作用与大洋岩石圈伸展构造的相互作用具有重要的理论意义.
A predominance of mantle peridotites over mafic rocks in the Tibetan Xigaze ophiolite distinguishes it from other classic ophiolites in the Tethyan belt, such as the Semail massif of Oman. Despite the abundance of new petrological, geochemical, geochronological and isotopic data on ophiolites that have accumulated over the last few decades, there is still no consensus on the tectonic setting and dynamic mechanisms of formation of these enigmatic bodies. This study involved detailed, large-scale mapping of the well-preserved Baimarang massif in the western part of the Xigaze ophiolite in order to provide key constraints on its tectonic setting. We demonstrate here that the Baimarang massif has a domal structure obviously different from most classic ophiolites. The massif is primarily cored by fresh harzburgites, locally containing small bodies of dunite, and dolerite intrusives. The fresh harzburgites have a well-developed foliation that dips northwest at angles of 60° to 80° and an associated spinel lineation. Mylonitic serpentinites invaded by dolerite intrusives occur on both the northern and southern sides of the fresh harzburgite. The transition between the fresh harzburgite and the mylonitic serpentinite is gradational and is marked by continuously increasing dolerite intrusives. The foliation in the northern mylonitic zone dips N to NW at angles of 50°-70°, whereas that in the southern belt dips SE at angles of 60°-70°. Most of the dolerite intrusives in the southern mylonitic belt are either irregular or lenticular in form. The shear sense in the dolerites indicates that the serpentinites were deformed by normal faulting, down to the south. In the upper part of the serpentinite belt a rare shear zone containing mylonitized harzburgite dips southwestward at angles of 50°-70°. The shear sense in this zone also indicates a southward movement. Partly surrounding the ultramafic core and serpentinite belt is a sequence of dolerite intrusives with an average thickness of 2 km, overlain by an extrusive sequence typically less than 500 m thick. The mafic rocks have undergone localized brittle deformation. On the basis of our detailed study of the Baimarang massif, we conclude that it consists of three zones： a core of fresh harzburgite that underwent plastic deformation at high temperature, a middle unit of mylonitic serpentinite characterized by ductile deformation, and an upper cover of mafic intrusives and extrusives showing only brittle deformation. We suggest that the structure of the Baimarang massif is analogous to that of oceanic core complexes along slow-spreading mid-ocean ridges or in forearc basins.
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第25期2823-2833,共11页
Chinese Science Bulletin
基金
中国地质调查局项目(12320115027701
121201102000150069)资助
关键词
日喀则蛇绿岩
蛇绿岩构造
大洋核杂岩
伸展构造
拆离断层
Xigaze ophiolite
structures of ophiolite
oceanic core complex
extensional structures
detachment
作者简介
E-mail：liyuancags@126．com
